A conventional star network includes a central node, (e.g., WiFi access point, WiMAX base station), and multiple leaf nodes (e.g., laptop computers) associated with the central node. The leaf nodes can only transmit to and receive from the central node directly. Communications between two leaf nodes associated with the same central node has to go through the central node. Therefore, the network forms a graph with a topology of a star.
A star network has many advantages. At most three nodes and two links (channels) are involved in any communication between any two nodes within the network. Therefore, performance is predictable.
The central node has total control of both access and resources. The central node coordinates the operations of all the leaf nodes in the network. Therefore, the central node can reduce or eliminate any collision and conflict between leaf nodes.
The capacity of the network can be expanded by only increasing capacity of the central node. The centralized nature also allows the monitoring of traffic through the network to determine unusual behavior. The simple star topology obviates the need for complex routing or message passing protocols.
One disadvantage of a start topology, however, is the dependence on the central node. The central node becomes the performance bottleneck, and can also be a cause of a single point of failure. The failure of the central node renders the entire network inoperable and therefore greatly reduces the network availability performance. In addition, the performance and scalability of the network depend on the capabilities of the central node.
Another disadvantage of the star topology is that each leaf node is connected to the network only by the link to the central node. If the individual link between a leaf node and central node becomes unreliable, the leaf node cannot communicate properly with other nodes in the network. The failure of an individual link between the center node and leaf nodes results in the isolation of the leaf node.
FIG. 1 shows a conventional wireless star network 100. The network includes a central node C 101, and leaf nodes Ln 102, where nε{1, 2, . . . , N}, where N is a maximal number of leaf nodes in the network. The central node and each leaf node are directly connected by a wireless link 120. The leaf nodes do not communication directly with each other. Communications between leaf nodes is via the central node. For instance, L1-C-L5 is the path for data transmitted from L1 to L5.
A standby central node can be added to the conventional star networks. The standby central node can enhance star network reliability and system availability. In the event of a failure of an active central node, the standby central node switches from standby mode to active mode, and resumes the operation of the failed active nodes and provides services with little or no interruption. The standby central node can also be available to assist data delivery between the active central node and the leaf nodes.
However, the standby central node has its disadvantages. During normal operation, the standby central node does not perform all the functions of the active central node. Its main role is to monitor the transmission by the active central node and leaf nodes. In the case of the active central node failure, it is not guaranteed that the standby central node can operate network properly. For example, the standby central node does not have full picture of the network. It does not know how many leaf nodes are in the network. There is no guarantee that the standby central node has reliable links with all leaf nodes in the network.
Another disadvantage of the standby central node is bandwidth inefficiency. The bandwidth allocated to the standby central node is wasted while network operates normally.
Another disadvantage is that the work load between the active central node and the standby central node is unbalanced while the network operates in normal mode.
FIG. 2 shows a conventional wireless star network 200 with active central node AC 101, a standby central node SC 201 and leaf nodes Ln 102. The active central node AC and each leaf node are directly connected by a wireless link 120. The standby central node SC and each leaf node are directly connected by a wireless link 220. Two central nodes 101, 201 are connected to each other by another link 204.